In a gas turbine engine, in order to achieve maximum engine efficiency (and corresponding maximum electrical power generation), it is important that the buckets rotate within the turbine casing or “shroud” with minimal interference and with the highest possible efficiency relative to the amount of energy available from the expanding working fluid. Typically, highest operation efficiencies can be achieved by maintaining a minimum threshold clearance between the shroud and tips of the bucket. Maintaining a minimum clearance prevents unwanted “leakage” of a hot gas over tip of the buckets, increased clearances lead to leakage problems and cause significant decreases in overall efficiency of the turbine. However, it should be appreciated that if bucket tips rub against a particular location of the shroud such that the bucket tip is eroded, the erosion of the bucket tip increases clearances between bucket tip and shroud in other locations, again resulting in unwanted leakage.
The need to maintain adequate clearance without significant loss of efficiency is made more difficult by the fact that as the turbine rotates, centrifugal forces acting on the turbine components can cause the buckets to expand in an outward direction toward the shroud, particularly when influenced by the high operating temperatures. Thus, it is important to establish the lowest effective running clearances between the shroud and bucket tips at the maximum anticipated operating temperatures.
Abradable type coatings have been applied to the turbine shroud to help establish a minimum, i.e., optimum, running clearance between the shroud and bucket tips under steady-state temperature conditions. In particular, coatings have been applied to the surface of the shroud facing the buckets using a material that can be readily abraded by the tips of the buckets as they turn inside the shroud at high speed with little or no damage to the bucket tips. Initially, a clearance exists between the bucket tips and the coating when the gas turbine is stopped and the components are at ambient temperature. Later, during normal operation the clearance decreases due to the centrifugal forces and temperature changes in rotating and stationary components inevitably resulting in at least some radial extension of the bucket tips, causing them to contact the coating on the shroud and wear away a part of the coating to establish the minimum running clearance. With abradable coatings clearances can be reduced with the assurance that if contact occurs, the sacrificial part is the abradable coating instead of the bucket tip.
Typically, the shrouds to which abradable coatings are applied to are fabricated (i.e. machined or cast) to include a concave profile that mates with a convex contour of a surface of the bucket tips (the rotation of the bucket tip will form a convex contour towards the shroud, though it should be appreciated that the surface of each bucket tip is not necessarily convex, and may be flat). Mating the concavely machined shroud with the convex bucket tip in this manner maintains a minimum clearance over the whole surface of the tip. Since an abradable coating applied to a concavely machined shroud includes the profile of the shroud to which it is applied, the abradable coating is also concavely disposed to mate with the convex tip. However, manufacturing a shroud to include the concave profile, or any desired profile, can be difficult and expensive. Thus, a method that would allow the abradable coating to include a profile that matches the profile of the bucket tips with which it interacts without machining the shroud is desirable.